1. Field of the Invention
The present invention relates to an apparatus for acquisition of an asynchronous, wideband DS/CDMA signal.
2. Description of the Related Art
An asynchronous, wideband, direct-sequence/code division multiple access (DS/CDMA) system is one of the next generation of mobile communication systems suggested in Japan and Europe. An asynchronous, wideband DS/CDMA system has an advantage that external timing information is not required, which is different from other DS/CDMA systems. In a conventional DS/CDMA system, the acquisition of code must precede the demodulation of data, however, in the case of an asynchronous system, base stations use different codes, so that it takes more time for the acquisition than in a synchronous system.
The asynchronous, wideband DS/CDMA system is a system that identifies channels or users by using a spreading code, so that its transmitter transmits signals of the modulated data multiplied by the spreading code. In the asynchronous, wideband DS/CDMA system, in order for a mobile terminal to demodulate the data transmitted from the base station, it must be preceded by an acquisition process. In a synchronous system such as the IS-95, which is now commonly used, all the base stations use the same codes and each base station is identified by a different offset, so that an acquisition process means a process for searching for the offset of the code used in the base station to which the mobile terminal belongs.
If the acquisition is not achieved, it is impossible to estimate the phases of channels. Therefore, generally a noncoherent detector, which can discriminate whether acquisition is achieved or not regardless of the phases of channels, is used in the acquisition process. FIG. 1 is a block diagram of a noncoherent detector. The noncoherent detector according to FIG. 1 includes an antenna 100, a local oscillator (LO) 102, a mixer 104, a correlator 106, a square multiplier 108, and a discriminator 110.
The operation according to the above structure is as follows. The antenna 100 receives a high frequency signal that has experienced fading and additive noise through radio channels. The mixer 104 multiplies the received signals by a signal produced in the local oscillator 102 and changes the received signal into a complex signal of base band. The correlator 106 correlates a real component and an imaginary component of the complex signal, respectively. The square multiplier 108 squares the correlated signal and removes a phase component induced by the channels. The discriminator 110 decides whether acquisition is achieved or not by discriminating the output values of the square multiplier 108.
The correlator 106 can be an active correlator or a matched filter correlator. An active correlator performs a correlation by multiplying codes generated by an internal code generator by the received signal and then by integrating the multiplied values over a correlation interval. Realization of the active correlator is relatively simple but the acquisition time thereof is long. The matched filter correlator has an advantage in that it takes a shorter time for acquisition than the active correlator, since the matched filter correlator can test different phases at every chip time.
FIGS. 2(a) through 2(e) illustrate the correlation results of the matched filter correlator for the case where acquisition is achieved and for the case where acquisition is not achieved, when the power of the received signal is 1 and ideal channels having no noise and fading are assumed. Symbol rk illustrates samples of the received signal at each tap of the matched filter. Symbol ck illustrates tap coefficients of the matched filter, and symbol pk illustrates the product of rk and ck.
FIG. 2(a) illustrates ck, the tap coefficients of the matched filter. FIG. 2(b) illustrates rk, the samples of the received signals when acquisition is achieved (in-sync), and FIG. 2(c) illustrates pk, the results of multiplying the values illustrated in FIGS. 2(a) and 2(b). According to these figures, if acquisition is achieved, all the pk values become 1 and the output value of the matched filter become 1.
FIG. 2(d) illustrates rk, the samples of the received signals when acquisition is not achieved (out-of-sync), and FIG. 2(e) illustrates pk, the results of multiplying the values illustrated in FIGS. 2(a) and 2(d). According to these figures, if acquisition is not achieved, the pk values become 1 or −1, and the sum thereof becomes much smaller than 1. In fact, due to the fading and additive noise of the channels, each of the output values of the matched filter becomes a complex number, and the outputs of the matched filter in the in-sync case can be smaller than the outputs of the matched filter in the out-of-sync case, which can result in a false lock.
In addition, if a signal-to-noise ratio is low, or the signal components are attenuated by the fading, it is impossible to make a reliable decision on whether acquisition is achieved or not only with the outputs of the matched filter of the received signal for the predetermined interval. Therefore, it can be more reliable to decide by combining the outputs of the matched filter obtained repeating for the above interval.
As a method for combining the outputs of the matched filters, there is a coherent combination method or a noncoherent combination method. The coherent combination method continuously accumulates the outputs of the matched filter during L intervals (where L is a positive integer), and then, squares the accumulated result and decides whether acquisition is achieved. However, performance of the coherent combination method is rapidly degraded if the fading or the offset of the frequency increases to more than a threshold value. The noncoherent combination method squares the outputs of the matched filter during the L intervals and linearly combines them to decide whether acquisition is achieved or not. That is, the output of the noncoherent combination method becomes the sum of the outputs of the noncoherent detector. However, performance of the noncoherent combination method is seriously degraded if the signal-to-noise ratio (SNR) worsens.